Sound velocity in liquid⁴He under pressure

We have measured the temperature dependence of the sound velocity in liquid⁴He at pressures from the vapor pressure to near the solidification pressure. The data were obtained at frequencies from 12 to 105 MHz and covered the temperature range from below 0.1 to nearly 1.0 K. The velocity data at high pressures do not show any anomaly that can be associated with a distinct shoulder found earlier in the sound attenuation. As observed previously at the vapor pressure, the frequency dependence of the velocity data undergoes an inversion at low temperatures; this unexpected behavior is in agreement with recent theoretical treatments.